Air bag passive restraint systems for protecting automobile and truck occupants in frontal collisions are beginning to be adopted by most of the world's automobile manufacturers. It has been estimated that by the mid-1990's all new cars and trucks manufactured will have air bag passive restraint sytems. These air bag systems are designed to protect occupants in frontal impacts only. Many people, however, are killed or seriously injured in side impacts, which typically involves one car running into the side of a second vehicle.
Approximately, one quarter of all injury-producing accidents in 1981, for example, were side impacts in which the direction of the force was determined to be within 45 degrees of the lateral axis of the vehicle. According to the National Highway Safety Council of the National Highway Traffic Safety Administration, 22% of all fatalities were caused by interior side surfaces of the vehicle as compared to 68% of fatalities caused by frontal impacts that includes the steering wheel, windshield frame, instrument panel, and windshield. Since air bags are well on their way to alleviating injuries from frontal impacts, it is important now to focus on the next largest killer, side impacts.
In frontal impacts, the crush zone of the vehicle changes velocity early in the crash and sensors located typically within 12 inches of the front of the vehicle can, in most cases, sense the crash and initiate the inflation of the air bag long before the occupant has begun to move relative to the passenger compartment. Also, for most cases, there is little intrusion in the passenger compartment and thus the entire space between the occupant and the instrument panel or the steering wheel is available to cushion the occupant. In contrast, in side impacts there is almost always significant intrusion into the vehicle and the motion of the occupant relative to the vehicle interior begins immediately after impact. In addition, there is far less space for a restraint system and thus the injury-reducing potential of an air bag even if it were deployed in time, is substantially less for side impacts than for frontal impacts. For frontal impacts, air bags are designed to cushion a 50 percentile male in a vehicle travelling at 30 MPH into a barrier. It is unlikely similar protection for an absolute velocity change of the passenger could be achieved in side impacts for for the reasons listed above without major modifications to vehicle side structures. Nevertheless, a significant percentage of side impacts occur at velocities low enough where an air bag could be of significant help in mitigating injuries.
To design a crash sensor for side impacts, the side-intrusion characteristics of a vehicle and the behavior of various parts of a vehicle during a side impact must be studied and fully understood. In a technical paper by V. Castelli and D. Breed, "Trends in Sensing Side Impacts," (SAE 890603) presented at the 1989 SAE Congress and Exposition in Detroit, the side intrusion problems and general vehicle response characteristics are discussed. In this paper, it is assumed that the marginal condition for an occupant to be critically injured occurs when the occupant impacts the side door panel at a critical speed "V-cr". A relative velocity of 10 to 12 miles per hour has typically been considered a reasonable threshold. Based on this criterion, the desired response curve of a side impact crash sensor can be determined by the impact conditions, such as vehicle-to-vehicle, vehicle-to pole, or truck-to-vehicle accidents. The following discussion, includes a brief summary of the aforementioned paper. For additional details, please refer to the paper.
The behavior of a struck vehicle depends on the striking object. Since most of the side impact accidents involve one vehicle impacting a second vehicle, consideration of this type of car-to-car crash is essential for the design of crash sensors. Define the struck vehicle as the "target" vehicle, and the striking vehicle as the "bullet" vehicle. For the discussion of side intrusion, consider the target vehicle comprising two parts: the side door beam and outer panel, and the passenger compartment. Once the side door is hit by the bullet vehicle, the door beam and outer panel deforms significantly while the passenger compartment only gains a relatively small velocity change in the early stage of a crash. The side intrusion or crush increases continuously after the early penetration until the entire car reaches a common final velocity later in a crash. The responses of the target and bullet vehicles are functions of the impact angle and location, the impact speed, and the stiffness and weights of the vehicles.
The velocity of the side door panel increases immediately after the impact to a maximum velocity comparable to the velocity of the bullet vehicle, V1. This rapid rise in velocity can happen within five to ten milliseconds. The passenger compartment experiences a relatively small velocity change during this stage of the crash. The difference in velocity between the side door and the passenger compartment manifests itself in the crush of the vehicle. As the side structure stiffens in the deep post-buckling range, the resistance force increases and starts to decelerate the side door panel until finally the side panel and the passenger compartment reach a common velocity, V2. This final velocity is estimated to be the momentum velocity, which is the original momentum of the bullet vehicle divided by the total masses of the bullet and target vehicles, assuming that the friction between the road surface and the vehicles is negligible and a perfectly plastic collision occurs. For two vehicles of the same mass and a 90.degree. impact of the moving bullet into a stationary target, the final velocity, V2, will be approximately equal to one half of V1.
Another critical parameter in the design of side impact sensor is the time when the occupant is hit by the side door inner panel, designated as t-hit. The deciding factors that influence t-hit are the stiffness of the vehicles, the impact condition, and the distance between the occupant and the side panel. In most of the side impact cases, t-hit occurs before the side door panel reaches the final velocity, V2. A crash sensor must trigger ahead of t-hit to allow the protective apparatus to deploy at an earlier time, defined as t-trigger. The gap between t-hit and t-trigger is the period needed for the inflatable system to deploy.
The velocity, at which the side door hits the occupant, is defined as V-hit. If the two vehicles are of equal weight in the 90.degree. impact described above, then the peak of velocity change of the side door panel can be as high as two times V-hit. For example, if V-hit is equal to 10 MPH, then the side panel of the target vehicle can experience a velocity rise up to about 20 MPH before it decelerates and finally reaches the final speed. In the marginally critical crashes, V-hit is equal to V-cr. This observation reveals that a crash sensor located at center points in the side door must require a velocity change higher than V-cr in an impulsive pulse, such as pulses in the range of 1-5 ms to trigger. Otherwise, if a crash sensor is responsive to an impulsive pulse with a velocity change of V-cr, then in many cases when the side door panel experiences a rapid velocity change of V-cr but finally a drop to one half of V-cr for equal mass vehicles, there will be many undesired initiations of the protection apparatus.
The above discussion on the velocity change, to which a crash sensor must respond, is based on the assumption that a velocity-type sensor is placed on the side door beam. A velocity-type sensor is a sensor which integrates a crash pulse and triggers when the velocity change exceeds a threshold value. Since a side impact crash sensor must not falsely trigger due to hammer blows or light pounding on the side door, which can cause significant local deformations on the side door, side door deformation can not be used as the only criterion for detecting the severity of a side impact accident. A side impact displacement type sensor which responds to the crush of the side door panel, therefore, could cause frequent inadvertent sensor triggering. A side impact sensor must also trigger for other side crashes when the side door is not directly hit but the impact is severe enough so that the occupant needs the protection of an inflatable system. A displacement-type sensor in these cases will not trigger until the side crush of the vehicle progresses to the location of the sensor. This will result in late triggering or no triggering of the sensor and no protection for the occupant. On the other hand, a velocity-type sensor will simply respond to the velocity change sensed in a crash, thus it can be adjusted to a desired sensitivity to predictably detect the occurrence of a side impact even though the side door is not hit directly. Based on the above observations, the velocity type sensor is appropriate for side impact inflatable systems. To ensure the effectiveness of sensing, it is reasonable that more than one sensor be used for side impact sensing, for example, one would be located just before the A-pillar, one just after the B-pillar, and one at the center of the side door. By implementing such a sensing system, it can be assured that at least one sensor will trigger for almost all side crashes in which the protection apparatus is needed.
Even though spring-mass inertial sensors also respond to a specified range of velocity changes, the sensitivity of these sensors increases as the pulse duration decreases. This means that these sensors will trigger with a smaller velocity change for pulses of shorter duration than longer duration. This trend contradicts the conditions of side impact sensing. On the other hand, viscously damped sensors, such as conventional ball-in-tube sensors, (as disclosed in U.S. Pat. Nos. 3,974,350, 4,198,864, 4,284,863 and 4,329,549 all to D. Breed) respond to the same velocity change regardless of pulse duration. These sensors also do not meet the requirements of side impact sensors, which requires greater insensitivity for short, impulsive velocity changes. In inertially damped sensors, the motion of the sensing mass is opposed by a nonlinear damping force, such as a resisting force depending on the second power of the velocity induced by fluid flow through a restrictor such as an orifice. These sensors are naturally more sensitive to long pulses than to short pulses, but the sensitivity to very long pulses can be compensated by a high bias force. The ability to tailor the characteristics of these sensors in the range of pulses 5 to 50 ms makes them most appropriate for side impact sensing.
A crash sensor for sensing side impacts must be placed on the side door structure to be effective. This location is essential since it is sensing the velocity change of the portion of the vehicle which will eventually strike the occupant and therefore serves as a good predictor of V-cr. If this sensor is placed on the door beam just inside the door outer panel, it will respond very quickly to the impact. If the sensor were placed at some other location in the vehicle, it would necessarily respond more slowly to a side impact into the door. Any crash sensor, to function properly, must be designed to operate either in the crush zone or out of the crush zone. Since there is insufficient signal anywhere else in the vehicle for side impacts, they can only be sensed in time with crush zone sensors. This sensor, therefore, must be in the appropriate crush zone in order to sense the crash in time. If the side door is not hit directly, the pulse propagated to the side door is delayed and stretched in its duration, as compared to the pulse generated in a direct side door impact. Therefore, to be effective, a crash sensor must be more sensitive to these longer or stretched pulses.
In another extreme case, such as hitting a soft cushion, the whole target vehicle may be subjected to a side velocity change while there is not penetration or deformation to the side door, and the occupant will move toward the side door and eventually hit the inside panel. Suppose V-cr is equal to 10 MPH and the gap between the occupant and the inside door panel is typically about 5 inches, then t-hit is approximately equal to 57 milliseconds assuming that the occupant travels with an average of 5 MPH from a zero initial velocity to a final 10 MPH speed. Notice that the side door panel and the passenger compartment in this case experience the same pulse. This indicates that at t-trigger, which is ahead of t-hit by a period needed for deployment, a sensor located on the side panel must respond to a velocity lower than V-cr. Even though these conditions are very rarely encountered, they can provide a guideline for the sensor design for pulses in the range of 30 to 45 milliseconds. For example, if a pulse of 50 milliseconds duration with a velocity change of 10 MPH is considered a marginal pulse, then the sensor will need to respond predictably to a velocity change of 6 to 8 MPH in the range of 30 to 40 milliseconds.
It may be desirable for a side impact sensing system to include safing (arming) sensors in addition to the discriminating sensors described above. In frontal impacts, velocity-type low-bias sensors located in the passenger compartment are used for safing purposes. In side impact crashes, however, the crash pulse in the passenger compartment does not provide enough information at the time when the crush zone sensor is required to trigger. Therefore, it is difficult to use a passenger-compartment safing sensor for a side impact sensing system. Safing sensors for side impact application could be crush sensing switches. These safing sensors should be placed in proximity to the velocity sensing sensors, and should have long contact dwells. A combination of a velocity sensing sensor and a crush sensor significantly reduces the probability of an inadvertent deployment by imposing a requirement that two environmental stimuli (velocity change and physical displacement) are required to initiate air bag deployment.